Issue removing Ngons from Interior
If low poly count is important then I would suggest you modify your workflow a little to something like the following:

First create your outer walls with window and door openings, then optimize this by merging vertices that are not doing anything to the ones that are. 



Next to create the inner walls use the solidify modifier. 

Apply this and then add the floor. 



Just realized that you said that you are only modeling interiors, so no need for solidify modifier.